1. Field of the Invention
The present invention relates to a process for the desulphurization of blister copper. More specifically, the invention is directed to an improved process of desulphurization of blister copper by introducing a mixture of an inert gas and an oxygen containing gas such as nitrogen and oxygen into liquid blister copper, and varying the amounts of oxygen or oxygen containing gas in the mixture during the process.
2. Description of the Related Art
The purpose of the fire-refining process is to lower the sulphur content of blister copper from about 0.05 to 0.005 wt. % and the oxygen content from about 0.3 to about 0.15 wt. %. Occasionally, the starting blister copper may have sulphur contents as high as 1 wt. % and extensive desulphurization is required. The first step involves the lowering of the sulphur content and it is usually called the desulphurization step. Desulphurization is then usually followed by a reduction step of the liquid copper. In most operations, the desulphurization process is carried out by blowing air into the liquid copper, forming SO2 gas which leaves the liquid copper. However, thermodynamically it is not necessary to use 21 vol. % oxygen in the gas to remove the sulphur and the high oxygen content in the injected gas, usually air, causes over-oxidation of the copper. This is particularly the case where stirring is poor.
During the initial stage of desulphurization, it may be suitable to use air with 21 vol. % oxygen. Since the amount of sulphur is fairly high, the oxygen will effectively be used to remove sulphur. However, during the latter stages of desulphurization, mass transfer of sulphur limits the rate of desulphurization. During this stage, the use of porous plugs with nitrogen containing about 5 vol. % oxygen will help to increase the stirring, as well as being sufficiently oxidizing to remove sulphur. This will reduce the amount of highly oxidized slag formed. In fact, nitrogen stirring at the end of the cycle may cause some of the copper oxide to back-react with sulphur thereby significantly improving the final metallurgy.
As seen in the paper by P. Goyal et al, xe2x80x9cGaseous Refining of Anode Copperxe2x80x9d, Journal of Metals, December 1982, pp. 22-28, the oxygen content in blister copper increases during the desulphurization step. Thermodynamically, in order to reach a sulphur content of 0.005 wt. % S in copper at the end of the desulphurization step, an inlet oxygen pressure of about 0.05 atm is required in order to prevent Cu2O formation at unit activity. With the injection of air, thermodynamically, Cu2O starts to form at a sulphur content of about 0.005 wt. %. In real practice, copper oxide starts to form at higher sulphur contents. The work by Goyal et al showed that it was possible to lower the sulphur content in copper by the use of nitrogen only. With nitrogen injection alone, the following reaction takes place.
2[O]+[S]+N2(gas)xe2x86x92SO2 in N2, PSO2 less than  less than 1 atmxe2x80x83xe2x80x83(1)
With the presence of nitrogen bubbles within the liquid copper, this reaction will proceed even if the equilibrium pressure of SO2 is very low. Thermodynamically it is known that the mass %0 in copper increases as the sulphur is removed, as shown by T. Shibasaki, T. Shimizu and N. Oguma (Mitsubishi Metal Corporation, Naoshima) in xe2x80x9cAnalysis of Process Dynamics and Improvement of Actual Operation of Anode Furnacexe2x80x9d, Int. Symp. Injection in Process Metallurgy, TMS 1991, Ed. T. Lehner, P. J. Koros and V. Ramachandran, pp. 265-276. At the final end-point of sulphur removal, the equilibrium SO2 pressure is approximately 0.05 atm. This means that to prevent over-oxidation, the final desulphurization stage should be done with a gas containing only about 5 vol. % oxygen.
The blister copper from the Mitsubishi process contains a high amount of sulphur with about 0.5 wt. % S. In order to prevent over-oxidation during desulphurization, Shibasaki et al used a mixture of steam and air instead of air only. They found that by the use of steam, the reduction time was decreased by 15 minutes and that the consumption of the reducing agent decreased by 13%. A second purpose for the use of a steam-air mixture was to decrease the tendency of tuyere plugging.
Fukunaka et al in xe2x80x9cDesulfuirization Kinetics of Molten Copper by Gas Bubblingxe2x80x9d, Metall. Trans. B., Vol. 22B, 1991, pp. 5-11, investigated the desulphurization kinetics of molten copper by using Arxe2x80x94O2 mixtures with 10, 20 and 30 vol. % of oxygen. They concluded that the overall reaction is composed of two elementary reactions, namely i) the desulphurization and ii) the dissolution rate of oxygen in copper. In their experimental set-up, basically all the oxygen was consumed by the desulphurization and dissolution reactions before the bubble broke through the surface. As oxygen was injected, there was a 10 seconds time delay before any SO2 evolved. This may be explained by the fact that initially the oxygen has to dissolve in the copper surrounding the gas bubbles. However, in their paper they do not mention anything about the resulting oxygen content in the copper and nothing about copper oxide formation.
In the case of Inco, S. W. Marcuson et al, xe2x80x9cPyrometallurgical Copper Refiningxe2x80x9d, U.S. Pat. No. 4,830,667, May 16, 1989, nitrogen is injected through commercially available fused alumina porous plugs during desulphurization of semi-blister copper. The purpose of the nitrogen injection is to stir the blister, allowing the air above the bath to react with sulphur in the semi-blister.
Rigby and Lanyi, xe2x80x9cUse of Porous Plugs in Molten Copper Production and Refiningxe2x80x9d, in Advances in Refractories for the Metallurgical Industries II Ed. M. Rigaud and C. Allaire, CIM 35th Annual Meeting in Montreal, August 1996, pp. 393-403, describe the use of porous plugs in various copper making steps. Although they stated that in some smelters, gas injection through porous plugs is being conducted with nitrogen having oxygen contents ranging between 0.2-3.0 vol. %, they did neither discuss the role of oxygen in the desulphurization process, nor mention or indicate that oxygen may react with sulphur to cause desulphurization.
It has also been observed that when a nitrogen-oxygen mixture is injected into blister copper, oxygen reacts according to
O2(in N2)+xS(dissolved)=xSO2(in N2)+yO(dissolved)xe2x80x83xe2x80x83(2)
This means that the pressure of SO2 in the gas within the liquid copper is always less than or equal to the pressure of oxygen in the injected gas. The lower the oxygen content in the injected gas, the lower the SO2 content in the gas bubbles leaving the copper during the desulphurization process. In addition, during desulphurization we have the following relationship                                                         [              S              ]                        +                          2              ⁡                              [                O                ]                                              =                                    SO              2                        ⁢                          xe2x80x83                        ⁢                          (              gas              )                                      ,                  K          =                                                    P                                  SO                  2                                                  (                  atm                  )                                                            %                ⁢                                  xe2x80x83                                ⁢                                  S                  ·                                                            %                      ⁢                                              xe2x80x83                                            [                      O                      ]                                        2                                                                        ≈            36                                              (        3        )            
which leads to                               %          ⁢                      xe2x80x83                    ⁢                      O            Cu                          ≈                  0.167          ·                                                    P                                  SO                  2                                                  (                  atm                  )                                                            %                ⁢                                  xe2x80x83                                ⁢                                  S                  Cu                                                                                        (        4        )            
This equation clearly shows that as the sulphur content decreases, the amount of dissolved oxygen in liquid copper increases for constant SO2 pressures. This is very important in terms of the amount of reduction which subsequently has to be done. If the sulphur is decreased to 0.003 wt. % at a SO2 pressure of 0.21 atm, the dissolved oxygen is 1.39 wt. %. However, if instead the equilibrium SO2 pressure had been 0.1 atm, the oxygen content would have been 0.96 wt. % and at 0.05 atm SO2, the amount of dissolved oxygen in the final copper is only 0.68 wt. %. This shows clearly that i) the required amount of oxygen decreases as the SO2 pressure decreases and ii) that the amount of reduction to be done also decreases. Since the SO2 pressure within the bubbles is directly related to the partial pressure of O2 in the inlet gas, a decrease in the inlet oxygen content will lead to less dissolved oxygen in the copper.
It is an object of the present invention to provide a process of desulphurization of blister copper which increases the recovery of copper, decreases the recycling of highly oxidized slag, decreases the time of desulphurization, decreases refractory wear, decreases subsequent time of reduction, and decreases the amounts of reactants used.
It is another object of the present invention to provide a process which improves the desulphurization of blister copper.
It is another object of the present invention to provide a process for the desulphurization of blister copper in which less over-oxidation takes place and the process is significantly improved.
It is yet another object of the present invention to provide a process of desulphurization of blister copper, with a nitrogen-oxygen containing gas of controlled and variable amounts of oxygen.
The invention relates to a process of desulphurization of blister copper during anode refining wherein a mixture of an inert gas and an oxygen containing gas is blown into liquid blister copper. The process is characterized by varying the amount of oxygen containing gas in the mixture during the desulphurization of blister copper. In practice, the inert gas is nitrogen, although carbon dioxide or argon and the like can also be used. The preferred inert gas is of course nitrogen. The oxygen containing gas may comprise oxygen per se or air. The usual mixture comprises nitrogen and air.
Preferably, the amount of oxygen the mixture is varied is about 21 to about 4 vol. %.
For example, the amount of oxygen in the mixture at the start of the desulphurization process may be set at about 21 vol. %, this amount being reduced to about 4 vol. % at the end of the desulphurization process.
Air is preferably used at the start of the desulphurization process. After an initial treatment with air, the desulfurization may be switched to a treatment with separate sources of nitrogen and oxygen wherein the amount of oxygen in the mixture is gradually decreased.
Decreasing the amount of oxygen relative to nitrogen may be provided by separately feeding from a liquid nitrogen tank and a liquid oxygen tank. Decreasing of the above amount may be carried step by step.
According to a preferred embodiment, at the end of the desulphurization process, porous plugs containing a mixture of nitrogen and oxygen wherein oxygen accounts for about 5 weight percent of the mixture, are introduced into the liquid blister copper, the plugs being handled to stir the liquid blister copper.
It has been found that the use of nitrogen-oxygen mixtures with oxygen contents varying for example from 21 to about 4 vol. %, instead of air, will improve the desulphurization of blister copper. Thermodynamically it is not necessary to use 21 vol. % oxygen in the gas to remove sulphur. The high oxygen content together with poor mixing in the vessel cause over-oxidation to take place. By using nitrogen with about 5 vol. % oxygen, less over-oxidation takes place and the process improves significantly. In particular, if such a gas mixture is used together with porous plugs, a much improved blister copper refining process will result.
The advantages of using such a varying amount of oxygen in the injected gas include
1) increased recovery of copper,
2) decreased recycling of highly oxidized slag,
3) decreased time of desulphurization,
4) decreased refractory wear
5) decreased subsequent time of reduction, and
6) decreased amount of reductants used.
It has also been realized that by using nitrogen with controlled amounts of oxygen during the desulphurization of blister copper during anode refining, the desulphurization process can be made more efficient in terms of i) decreasing the degree of over-oxidation of the copper, ii) increasing the recovery of copper during the desulphurization process, iii) reducing the requirement of de-oxidation, iv) shorter cycle time and v) reduced refractory corrosion. This will lead to increased productivity and improved profitability.